Use select() syscall to avoid busy waiting

Currently it consumes a lot of CPU, it's a no-brainer i think.

Co-authored-by: Diego de Estrada

src/Transport/Curl.php

@@ -245,6 +245,10 @@ public function request_multiple($requests, $options) {
 		do {
 			$active = 0;
 
+			if ($active) {
+				curl_multi_select($multihandle);
+			}
+
 			do {
 				$status = curl_multi_exec($multihandle, $active);
 			}